1. Field of the Invention
The invention relates to a method for improving the quality of abrasive grains of Al.sub.2 O.sub.3 comprising titanium oxide, preferably Ti.sub.2 O.sub.3, whereby a ceramic coating is applied, e.g. melted and/or sintered, onto the abrasive grain.
The invention also relates to the treatment of conventional abrasive grains of molten or sintered electrocorundum, which are commercially available as normal corundum or microcrystalline corundum and contain between 0.5 and 5.0 weight percent of titanium oxide (Ti.sub.2 O.sub.3 and TiO.sub.2), the chief constituent being Ti.sub.2 O.sub.3.
2. Description of the Prior Art
Corundum, which is to be used as an abrasive means in abrasive articles, such as abrasive disks, is, according to prior art methods, occasionally annealed at a temperature of about 1350.degree. C., whereby the grain structure is changed and the abrasive properties are improved. This treatment shows in the blue colour which the corundum takes on in the course of this treatment.
It is further known in the art to provide corundum grains with a coating, e.g. of ceramic materials and ferric oxides, producing an improved adhesion between the abrasive grain and the bond in the abrasive article resp. having filler properties and taking an active part in the abrasive process, e.g. by improving the heat removal.
In prior art methods the abrasive grain is annealed and coated in two separate operating cycles. First the abrasive grain is heated in oxidizing atmosphere to a temperature of about 1350.degree. C. until it takes on blue colour and then the abrasive grain is cooled off again to room temperature. After cooling off, a ceramic coating is applied onto the abrasive grain in a separate operating cycle, said coating being based on phosphates or silicates with low-melting, ceramic frits (glass frits) and fine ground metallic oxides--preferably Fe.sub.2 O.sub.3 for cost saving reasons. For this purpose the abrasive grain is heated again to a temperature of about 600.degree. C. to 800.degree. C.
The thus obtained coating creates a greater surface and a better surface wettability due to its rough, granular surface structure. Consequently, a greater and improved adhesive surface is provided for the resinoid bond of the abrasive article, whereas the adhesion between the abrasive grain and the coating is frequently not as intense as required. When abrasive grains treated in the above-described manner are inserted into the abrasive article, grains break off easily, which have not or only insufficiently been utilized in the abrasive process, particularly in the case of abrasive disks of high density. A further reason for the breaking off of abrasive grains is the porous and relatively thick (15-50 .mu.m) coating itself which is only of low strength. The liquid constituent of the resinoid bond is not always able to soak the thick coating entirely, i.e. to its bottom. Therefore, the porous coating itself forms a preferred fracture zone in the course of subsequent stress. Even if good adhesion is obtained between the coating and the grain and between the coating and the bond, the coating itself can be torn, whereby one part of the coating adheres to the grain, and the other part adheres as corresponding part to the binding material of the abrasive article.